Steam cracking has long been used to crack various gaseous (e.g., light alkanes) or liquid (e.g., naphthas) hydrocarbon feedstocks into higher value products, such as olefins, preferably light olefins such as ethylene and propylene. In addition to naphthas, other liquid feedstocks of interest may include, for example, distillation resids or bottoms, gas oils, kerosenes, crudes, various other liquid separation product streams, and blends thereof. When cracking liquid feedstocks having final boiling points higher than naphthas, the steam cracking process often produces numerous by-products, such as various aromatic compounds, ash, metals, coke, asphaltenes, and other high weight materials including molecules that tend to combine to form high molecular weight materials commonly known as tar. Similarly, cracking heavier liquid feedstocks (e.g., feeds having a final boiling point above 260° C.) generally produce more tar than lighter liquid feeds such as naphthas. Tar is a high-boiling point, viscous, reactive material comprising many complex, ringed and branched molecules which can polymerize and foul equipment under certain conditions. Tar also typically contains high-boiling and/or non-volatile components including paraffin-insoluble compounds, such as pentane-insoluble (PI) compounds or heptane-insoluble (HI) compounds, which are molecules of high molecular weight, multi-ring structures, collectively referred to as asphaltenes. Asphaltenes content can progress for a time under various post-cracking conditions, particularly as the steam cracker effluent cools, particularly accelerated as the tar-containing effluent cools below 300° C. The term “final boiling point above X” means that at temperature X, a sample of the material still exhibits at least some non-volatized portions, at least a further portion of which may still be volatized at a temperature greater than X.
Tar and associated asphaltenic materials can precipitate buildup in, and plug piping, vessels, and related equipment downstream of the cracking furnace. Further, asphaltenic materials reduce the economic value and further processability of tar by rendering the tar highly viscous and less compatible for mixing or blending with highly paraffinic streams or for use with fuel streams. When so blended, the paraffinic streams and asphaltenes can further induce precipitation of the paraffin-insoluble components in the resulting mixture. Various methods are known in the art to treat tars produced from steam cracking liquid feedstocks.
U.S. Pat. No. 3,691,058, incorporated herein by reference in its entirety, discloses an integrated visbreaking-hydrocracking process to break down steam cracker tars into single-ring aromatics.
U.S. Pat. No. 3,707,459, incorporated herein by reference in its entirety, discloses visbreaking residua, e.g., thermal tar from steam cracking, in the presence of free radical acceptors, e.g., CaO, isooctane, and n-heptane.
U.S. Pat. No. 4,575,413, incorporated herein by reference in its entirety, discloses adding aluminum salts to reduce fouling in steam cracker tar streams.
DE 4308507 discloses reducing viscosity of heavy oil residues by treatment at high temperature (427° C.) with a hydrogen donor solvent comprising a fuel oil from steam cracking, which contains hydroaromatic compounds.
U.S. Pat. No. 5,215,649, incorporated herein by reference in its entirety, discloses producing gaseous olefins by cracking a hydrocarbon feedstock stream wherein the cracked product stream is quenched to stop cracking, followed by injecting hydrogen donor diluent, e.g., dihydronaphthalenes, which suppress molecular weight growth reactions forming undesirable high molecular weight materials such as asphaltenes.
U.S. application Ser. No. 12/023,204, filed Jan. 31, 2008, discloses upgrading steam cracker tar by heating from below 300° C. to a temperature above 300° C. for a time sufficient to convert at least a portion of the steam cracker tar to lower boiling molecules.
U.S. application Ser. No. 12/099,971, filed Apr. 9, 2008, discloses upgrading steam cracker tar by reheating the tar from temperatures below 300° C. to a temperature above 300° C. in the presence of steam for a time sufficient to convert at least a portion of the steam cracker tar to lower boiling molecules and subsequently separating the reheated steam cracker tar into one or more a tar-lean products and a tar-rich product boiling above the tar-lean products. However, the '971 invention primarily addresses reducing previously formed steam cracker tar, after the effluent has cooled for sufficient time to permit tar precipitation and polymerization.